For very high frequencies, electrical signal processing must partly be passively performed since signal processors are not available that in real time can execute all required operations. Some advanced signal processing can for example be executed in discrete components, for example SAW-components, or in some cases in microwave components that can be manually tuned. These components are more or less costly and/or require much space.
Furthermore, in fast electronic circuits it is often necessary to use delay lines to adapt the clock signal to the signal carrying information. It is conventionally produced by additional lengths of lines, i.e. by having the signal pass a line of extra large length, that often requires is much space on a circuit board and in addition complicates the layout of lines for connection to and between other components on the same circuit board.
It is generally true that it is desirable to select for a transmission line a dielectric having a low dielectric constant to obtain the least possible delay. However, in the case where one tries to accomplish couplers directly in a wire pattern, they could be made much smaller if it would be possible to produce a limited area having a higher dielectric constant at exactly the place where the coupler is located.
In many applications there is no problem associated with delay but instead problem can exist associated with the often high line density producing cross-talk between adjacent lines.
When accomplishing impedance adaptation for lines on commonly used substrates having a constant dielectric constant the widths of the strip shaped conductors used must be exponentially increasing when an adaptation to lower impedances is to be made. The widths of the conductors are often much large than the widths of the components to which the conductors are to be connected and thus the impedance adaptation is only virtual.
In the optical domain patterned variations of thickness have been used to produce Bragg filters, then using material grown in the height direction, and variations in refractive index to produce Bragg mirrors.
A method of producing Bragg mirrors in the electrical domain comprises stepwise changing the width of a conductor. It gives peculiar results since the configuration of the electromagnetic field does not directly adapt itself to different conductor widths.
In electrical systems a unique possibility exists to vary the signal velocity stepwise without obtaining reflections by varying at the same time the dielectric constant and the width of the conductors, maintaining the same characteristic impedance.
Variation of the electrical constant of a dielectric surrounding a conductor, or located at a conductor, has been proposed in U.S. Pat. No. 5,796,317 and the published Japanese patent application 7074506. In this Japanese patent application is disclosed how a filter in a transmission line can be produced by periodically changing, instead of designing the transmission line to have some periodically repeated wider portions, the dielectric constant of the material at the conductor path of the transmission line.
In the published European patent application 0 343 771 different methods are disclosed for producing waveguides using a porous, compressible dielectric material. In one embodiment a groove can be formed in a material which is filled with material of a higher dielectric constant.